1. Field of the Invention
Embodiments of the present invention generally relate to method and apparatus for processing a substrate. Particularly, embodiments of the present invention relate to methods and apparatus for monitoring dosages of one or more species during plasma processing of semiconductor substrates.
2. Description of the Related Art
It is important to control ion dosage during plasma processes, such as plasma enhanced chemical vapor deposition (PECVD) process, high density plasma chemical vapor deposition (HDPCVD) process, plasma immersion ion implantation process (P31), and plasma etch process. Ion implantation processes in integrated circuit fabrication particularly require instrumentation and control to achieve a desired ion dose on a semiconductor substrate.
The dose in ion implantation generally refers to the total number of ions per unit area passing through an imaginary surface plane of a substrate being processed. The implanted ions distribute themselves throughout the volume of the substrate. The principal variation in implanted ion density (number of ions per unit volume) occurs along the direction of the ion flux, usually the perpendicular (vertical) direction relative to the substrate surface. The distribution of ion density (ions per unit volume) along the vertical direction is referred to as the ion implantation depth profile. Instrumentation and control systems for regulating ion implant dose (ions per unit area) are sometimes referred to as dosimetry.
Ion implantation may be performed in ion beam implant apparatus and in plasma immersion ion implantation apparatus. Ion beam implant apparatus, which generate a narrow ion beam that must be raster-scanned over the surface of the substrate, typically implant only a single atomic species at one time. The ion current in such an apparatus is precisely measured and integrated over time to compute the actual dose. Because the entire ion beam impacts the substrate and because the atomic species in the beam is known, the ion implant dose can be accurately determined. This is critical in an ion beam implant apparatus, because it employs a D.C. ion source, which is subject to significant drift in its output current, and the various grids and electrodes employed in the beam implant machine drift as well (due to the susceptibility of a D.C. source to accumulation of deposited material on component surfaces). Accordingly, precise dosimetry is essential in an ion beam implant apparatus. The precisely monitored ion beam current is integrated over time to compute an instantaneous current implant dose, and the process is halted as soon as the dose reaches a predetermined target value.
In contrast, plasma immersion ion implantation reactors present a difficult problem in dosimetry. Typically, the atomic weight of the ions incident on the substrate cannot be precisely determined because such a reactor employs a precursor gas containing the desired ion implantation species as well as other species. For example, since pure boron is a solid at room temperature, plasma immersion ion implantation of boron must employ a multi-species gas such as B2H6 as the plasma precursor, so that both boron and hydrogen ions are incident on the substrate. As a result, determining the boron dose from a measured current is difficult. Another difficulty in implementing dosimetry in a plasma immersion ion implantation reactor is that the plasma ions impact the entire substrate continuously, so that it is difficult to effect a direct measurement above the substrate of the total ion current to the substrate. Instead, the dose must be indirectly inferred from measurements taken over a very small area. A further difficulty encountered in some plasma immersion ion implantation reactors is the presence of electromagnetic noise or interference in the chamber that can prevent a precise measurement of ion current. This is particularly true of reactors employing RF plasma source power or RF plasma bias power.
Plasma immersion ion implantation reactors employing D.C. (or pulsed D.C.) plasma source power are susceptible to drift in the plasma ion current due to deposition of material on internal reactor components from the plasma. Such reactors therefore require precise real-time dosimetry. This problem has been addressed by providing a small orifice in the wafer support pedestal or cathode outside of the substrate periphery, for plasma ions to pass through into the interior volume of the cathode. An electrode sometimes referred to as a Faraday cup faces the orifice and is biased to collect the ions passing through the orifice. The interior of the cathode can be evacuated to a slightly lower pressure than the plasma chamber to ensure efficient collection of ions through the orifice. A current sensor inside the cathode interior measures the current flowing between the ion-collecting electrode and its bias source. This current can be used as the basis of a dosimetry measurement. One problem with such an arrangement is that the current measurement cannot distinguish between different atomic species, and therefore cannot provide an accurate measurement of the species of interest (e.g., boron). Another problem is that the transmission of the measured current from the current sensor inside the cathode interior to an external controller or processor can be distorted by the noisy electromagnetic environment of the plasma reactor.
Another problem is that the orifice in the cathode constitutes an intrusion upon the ideal plasma environment, because the orifice can distort the electric field in the vicinity of the substrate periphery. Furthermore, plasma passing through the orifice can cause problems by either sputtering the orifice surfaces or by depositing on the orifice interior surfaces, requiring the periodic cleaning of the orifice interior.
In plasma immersion ion implantation reactors employing RF plasma source power, precise or real-time dose measurement typically was not critical. This is due in part to the fact that an RF plasma is relatively impervious to deposition of material on internal chamber components, so that the ion flux at the wafer surface does not drift significantly, compared to a reactor employing a D.C. plasma source. Moreover, real-time dose measurement in such a reactor is difficult. For example, the harsh RF environment of such a reactor would distort an ion current measurement taken inside the cathode (as described above) as it is conveyed to an external controller or processor. To avoid such problems, implant dose can be reliably controlled based upon the predicted or estimated time required to reach the target implant dose. However, a real-time does control is more and more in need as the feature size becomes smaller and smaller in the semiconductor devices.
Therefore, there is a need for precise real-time dosimetry in a plasma processing chamber, such as an RF plasma immersion ion implantation reactor.